1. Field of the Invention
The present invention relates to an acoustic apparatus comprising a resonator or using a resonator as an acoustic radiation member.
2. Prior Art
A speaker system as one type of acoustic apparatus is arranged such that a speaker unit (vibrator) is disposed in a cabinet and is driven by an amplifier (AMP). Of reproduction characteristics of the speaker system, low-frequency reproduction characteristics are mainly determined by the volume of the cabinet.
When a dynamic direct radiator speaker (dynamic cone speaker) is used in an acoustic apparatus, a direct sound is radiated from the front surface of the diaphragm, and acoustic waves are also radiated from its rear surface. The phase of the acoustic waves from the front and rear surfaces are opposite to each other. Therefore, if a difference in propagation distance of the acoustic waves from the front and rear surfaces to a listener is almost an odd multiple of a half wavelength, sound pressures from these surfaces are in phase with each other, and are superposed.
However, if the difference in propagation distance of the acoustic waves is almost an even multiple of the half wavelength, the sound pressures cancel each other and are attenuated. Thus, taking into consideration the fact that sounds having various wavelengths are radiated from the speaker, it is preferable that the sound from the rear surface does not reach the listener or does not adversely influence the direct radiation sound from the front surface.
For this purpose, the direct radiation speaker employs a baffle. As a baffle for shielding communication of sounds from the front and rear surface of the diaphragm, a plane baffle, back-opening cabinet type baffle, closed baffle, and the like are known. Furthermore, as a baffle having a slightly different purpose from the above baffles, a phase inversion type (bass-reflex type) baffle is known. (In this specification, these baffles are referred to as first to fourth prior arts, respectively.)
In such conventional acoustic apparatuses described above, various countermeasures are taken in order to allow low-frequency reproduction.
The plane baffle, back-opening baffle and closed baffle are designed such that radiation sounds from the rear surface of the diaphragm do not reach a listener in front of the speaker system as unnecessary sounds. However, in order to improve the bass reproduction characteristics with these baffles, the apparatus (cabinet) will inevitably be made large in size, and even if it is made so to a certain feasible extent, its low-frequency reproduction characteristics will be insufficient.
In the bass-reflex type speaker system, the phase of the backward sound is inverted by the opening port, so that, in particular, a bass range of a direct radiation sound from the front surface of the diaphragm is compensated for. However, at that time, the resonance system which is originally very hard to deal with is undesirably formed on the two portions, i.e., the diaphragm and the opening port. In order to obtain a satisfactory bass-reflex effect according to the standard setting, the optimal condition of the system must be very critically set while taking the mutual dependency condition of these two resonance systems. Although various attempts have been made in this respect as disclosed in Japanese Patent Publication No. sho 46-12670 and Japanese Utility Model Publication No. sho 54-35068, these attempts could not eliminate difficulty on design.
Whether the optimal design of said speaker system has been achieved or not, the cabinet undesirably becomes bulky in order to improve the low-frequency reproduction characteristics.
Therefore, when a bass reproduction capability of a certain level or higher is to be obtained according to any of the prior arts, the resulting cabinet will inevitably become large in size. As a result, it is difficult to employ an acoustic apparatus having a cabinet of a proper volume and excellent low-frequency reproduction characteristics in a variety of applications such as in halls, rooms, vehicles, and the like.
As is so in the bass-reflex speaker system described above, in an acoustic apparatus, a resonance phenomenon is utilized in a variety of forms.
There has been known, as a fifth prior art, an acoustic apparatus comprising a resonator partitioned into two chambers A and B by a partition wall, and a dynamic electroacoustic transducer (dynamic speaker) serving as a vibrator and being attached to a hole formed in the partition wall. In this acoustic apparatus, opening ducts are provided respectively to the chambers A and B, and resonance acoustic waves are radiated outwards from these ducts. The chambers A and B respectively have resonance frequencies f.sub.oa (Hz) and f.sub.ob (Hz) determined by the volumes of cavities (i.e. the volumes of chambers A and B), the dimensions of the opening ducts, and the like. Therefore, when the speaker is driven by an amplifier or the like, in the chambers A and B, a resonance phenomenon occurs by the vibration of the diaphragm of the speaker, and an output energy at that time has maximum values near the above-mentioned resonance frequencies. As a result, there can be obtained the resonance acoustic waves having sound pressure-frequency characteristics having peaks at said respective frequencies f.sub.oa and f.sub.ob.
There has been also known, as a sixth prior art, an acoustic apparatus comprising a resonance chamber defined by a cabinet, a first dynamic electro-acoustic transducer (speaker) serving as a vibrator and being attached to the resonance chamber, and an opening, formed in the resonance chamber, for radiating outwards a resonance acoustic wave. A second dynamic electro-acoustic transducer (speaker) is separately provided to said cabinet, so that an acoustic wave is directly radiated outwards therefrom. In this acoustic apparatus, when the first speaker is driven by an amplifier, a resonance phenomenon occurs in the resonance chamber due to the vibration of the diaphragm of the first speaker. Therefore, separately from the direct radiation by the second speaker, acoustic reproduction is made from the opening to have a peak sound pressure near a resonance frequency f.sub.o inherent in the resonance chamber.
However, according to the conventional acoustic apparatuses, the vibrator undesirably causes a decrease in resonance Q value of the resonator serving as an acoustic radiation member. This is because the speaker as the vibrator has an inherent internal impedance Z.sub.v, and the internal impedance acts as an element which damps the resonance of the resonator. In this manner, as the resonance Q value becomes low, radiation capability of the resonance acoustic wave becomes inevitably low, and the presence of the resonator in the acoustic apparatus becomes meaningless.
If the resonance frequency is lowered while rendering the resonator compact, the opening duct must be elongated. Accordingly, the acoustic resistance (mechanical resistance) of the opening duct is inevitably increased, and the resonance Q value is decreased further. For this reason, the acoustic radiation capability is further decreased due to the decrease in the resonance Q value, and the acoustic apparatus is not suitable for a practical use.
As a result, any of the conventional apparatuses does not have sufficient resonance radiation capability. If a certain level of capability is to be maintained, the resulting cabinet will inevitably be made extremely large in size.